Practical organic chemistry 4Classification of organic

compounds by solubility testsPrepared by :

Pshtiwan Ghareeb Ali

Bsc. In Pharmacy

IntroductionLike dissolves like; a substance is most soluble in that solvent to which it is most closely related in structure.

When a solid or liquid dissolves, the structural units-ions or molecules become separated from each other, and the spaces in between become occupied by solvent molecules. In dissolution, as in melting and boiling, energy must be supplied to overcome the interionic or intermolecular forces. Where does the necessary energy come from?

The energy required to break the bonds between solute particles is supplied by the formation of bonds between the solute particles and the solvent molecules: the old attractive forces are replaced by new ones. Now, what are these bonds that are formed between solute and solvent?

In the case of ionic solutes

A great deal of energy is necessary to overcome the powerful electrostatic

forces holding together an ionic lattice. Only water or other highly polar

solvents are able to dissolve ionic compounds appreciably.

Polar molecule has a positive end and a negative end. Consequently, there is

electrostatic attraction between a positive ion and the negative end of the

solvent molecule, and between a negative ion and the positive end of the

solvent molecule. These attractions are called ion-dipole bonds. Each ion-

dipole bond is relatively weak, but in the aggregate they supply enough

energy to overcome the interionic forces in the crystal.

In solution each ion is surrounded by a cluster of solvent molecules, and is said to be solvated;

if the solvent happens to be water, the ion is said to be hydrated. In solution, as in the solid and liquid states, the unit of a substance like sodium chloride is the ion, although in this case it is a solvated ion.

To dissolve ionic compounds a solvent must also have a high dielectric constant, that is, have high insulating properties to lower the attraction between oppositely charged ions once they are solvated. Water owes its superiority as a solvent for ionic substances not only to its polarity and its high dielectric constant but to another factor as well: it contains the -OH group and thus can form hydrogen bonds. Water solvates both cations and anions: cations, at its negative pole (its unshared electrons, essentially); anions, through hydrogen bonding.

what is the difference between dissolving and reacting?

Now let us turn to the dissolution of non-ionic solutes.

The solubility characteristics of non-ionic compounds are determined chiefly by their polarity. Non-polar or weakly polar compounds dissolve in non-polar or weakly polar solvents; highly polar compounds dissolve in highly polar solvents.

To apply the rule “Like dissolves like,” you must first determine whether a substance is polar or nonpolar. The polarity of a compound is dependent on both the polarities of the individual bonds and the shape of the molecule.

Precise description of solubility

Solubility can be described more precisely in terms of the extent to which a substance is soluble. Solubility may be expressed in terms of grams of solute per liter (g/L) or milligrams of solute per milliliter (mg/mL) of solvent Consider the solubilities at room temperature for the following three substances in water:

For the present purposes, a compound is defined as soluble if it dissolves to the extent of 3 g in 100 mL of water or other solvents.

Important of solubility and solubility tests

• The solubility of a solute (a dissolved substance) in a solvent (the dissolving medium) is the most important chemical principle underlying three basic techniques you will study in the organic chemistry laboratory: crystallization, extraction, and chromatography.

• The presence of a functional group is often indicated.

• Certain deductions about molecular size and composition may sometimes be made.

• Polarity, acidity and basicity also can be determined.

Miscible, Immiscible

When the solubility of a liquid solute in a solvent is described, it is sometimes helpful to use the terms miscible and immiscible.

Two liquids that are miscible will mix homogeneously (one phase) in all proportions. For example, water and ethyl alcohol are miscible. When they are mixed in any proportion, only one layer will be observed .

Miscible, Immiscible

Two immiscible liquids do not mix homogeneously in all proportions, and under some conditions they will form two layers. Water and diethyl ether are immiscible. When mixed in roughly equal amounts, they will form two layers.

The terms solubility and miscibility are related in meaning, it is important to understand that there is one essential difference. There can be different degrees of solubility, such as slightly, partially, very, and so on. Unlike solubility, miscibility does not have any degrees—a pair of liquids is either miscible, or it is not.

For the present purposes, a compound is defined as soluble if it dissolves to the extent of 3 g in 100 mL of water or other solvents.

Guidelines for Predicting Polarity and Solubility:FOR Solutions in Which the Solvent and Solute Are Molecular

1. All hydrocarbons are nonpolar.Examples:

Hydrocarbons such as benzene are slightly more polar than hexane because of their pi bonds, which allow for greater van der Waals or London attractive forces.

2. Compounds possessing the electronegative elements oxygen or nitrogen are polar.

The polarity of these compounds depends on the presence of polar C—O, C=O,

OH, NH, and CN bonds. The compounds that are most polar are capable of forming

hydrogen bonds (see Guideline 6) and have NH or OH bonds. Although all of

these compounds are polar, the degree of polarity ranges from slightly polar to

highly polar. This is due to the effect on polarity of the shape of the molecule and

size of the carbon chain, and whether the compound can form hydrogen bonds.

3. The presence of halogen atoms, even though their electronegativitiesare relatively high, does not alter the polarity of an organic compound in a significant way. Therefore, these compounds are only slightly polar. The polarities of these compounds are more similar to those of hydrocarbons , which are nonpolar, than to that of water, which is highly polar .

4. When comparing organic compounds within the same family, note that adding carbon atoms to the chain decreases the polarity. For example, methyl alcohol (CH3OH) is more polar than propyl alcohol (CH3CH2CH2OH). The reason is that hydrocarbons are nonpolar, and increasing the length of a carbon chain makes the compound more hydrocarbon-like.

5. Compounds that contain four or fewer carbons and also contain oxygen or nitrogen are often soluble in water. Almost any functional group containing these elements will lead to water solubility for low-molecular-weight (up to C4) compounds. Compounds having five or six carbons and containing one of these elements are often insoluble in water or have borderline solubility

6. As learned before, the force of attraction between polar molecules is dipole–dipole interaction. A special case of dipole–dipole interaction is hydrogen bonding. Hydrogen bonding is a possibility when a compound possesses a hydrogen atom bonded to a nitrogen, oxygen, or fluorine atom. The bond is formed by the attraction between this hydrogen atom and a nitrogen, oxygen, or fluorine atom in another molecule. Hydrogen bonding may occur between two molecules of the same compound or between molecules of different compounds , When hydrogen bonding between solute and solvent is possible, solubility is greater than one would expect for compounds of similar polarity that cannot form hydrogen bonds.

7. Another factor that can affect solubility is the degree of branching of the alkyl chain in a compound. Branching of the alkyl chain in a compound lowers the intermolecular forces between the molecules. This is usually reflected in a greater solubility in water for the branched compound than for the corresponding straight-chain compound. This occurs simply because the molecules of the branched compounds are more easily separated from one another

8. The solubility rule (“Like dissolves like”) may be applied to organic compounds that belong to the same family. For example, 1-octanol (an alcohol) is soluble in the solvent ethyl alcohol. Most compounds within the same family have similar polarity. However, this generalization may not apply if there is a substantial difference in size between the two compounds. For example, cholesterol, an alcohol with a molecular weight (MW) of 386.64, is only slightly soluble in methanol (MW 32.04). The large hydrocarbon component of cholesterol negates the fact that they belong to the same family.

9. Almost all organic compounds that are in the ionic form are water soluble (see next - Solutions in Which the Solute Ionizes and Dissociates).

10. The stability of the crystal lattice also affects solubility. Other things beingequal, the higher the melting point (the more stable the crystal), the less soluble the compound. For instance, p-nitrobenzoic acid (mp 242°C) is, by a factor of 10, less soluble in a fixed amount of ethanol than the ortho (mp147°C) and meta (mp 141°C) isomers.

Notice that the para isomer has the highest melting point and the lowest solubility, even though the polarities of all three isomers are similar

Compounds in Increasing Order of Polarity

Solvents in Increasing Order of Polarity

Guidelines for Predicting Polarity and Solubility:FOR Solutions in Which the Solute Ionizes and Dissociates

Many ionic compounds are highly soluble in water because of the strong attraction between ions and the highly polar water molecules. This also applies to organic compounds that can exist as ions. For example, sodium acetate consists of Na+ and CH3COO– ions, which are highly soluble in water. Although there are some exceptions, you may assume that all organic compounds that are in the ionic form will be water soluble.

The most common way by which organic compounds become ions is in acid–base reactions. For example, carboxylic acids can be converted to water soluble salts when they react with dilute aqueous NaOH:

The water-soluble salt can then be converted back to the original carboxylic acid (which is insoluble in water) by adding another acid (usually aqueous HCl) to the solution of the salt. The carboxylic acid precipitates out of solution. Amines, which are organic bases, can also be converted to water-soluble salts when they react with dilute aqueous HCl:

This salt can be converted back to the original amine by adding a base (usually aqueous NaOH) to the solution of the salt.

Common Organic Solvents

Solubility in water

• It must be of low molar mass and will usually contain no more than four to five carbon atoms, unless it is polyfunctional.

• It must contain a polar group that will form a hydrogen bond with water, such as the hydroxy group of an alcohol or a carboxylic acid, the amino functionality of an amine, or the carbonyl group of aldehydes or ketones. Esters, amides, and nitriles dissolve to a lesser extent.

• alkanes, alkenes, alkynes, and alkyl halides are water-insoluble.

Solubility in Aqueous Sodium hydroxide

• Carboxylic acids, which are strong acids, and phenols, which are weak acids, dissolve in sodium hydroxide because they are converted into their water-soluble sodium salts.

Solubility in Aqueous Sodium bicarbonate

If the compound is soluble, the tentative conclusion is that a carboxylic acid group is present, owing to the formation of the water-soluble sodium salt ; phenols are normally not deprotonated in this medium. Dissolution should be accompanied by effervescence resulting from decomposition of the carbonic acid, H2CO3, formed from reaction of bicarbonate with the carboxylic acid, to carbon dioxide and water.

Solubility in Aqueous Sodium bicarbonate

Phenol containing one or more strong electron-withdrawing substituents, such as a nitro group, can be as acidic as a carboxylic acid and thus may form a water-soluble sodium salt by reaction with NaHCO3. Similarly, the salt of a carboxylic acid of high molar mass may not be completely soluble in the aqueous medium; nevertheless, evolution of CO2 should still be observable when the acid comes in contact with the base.

Solubility in Aqueous Hydrochloric acid

If the unknown is soluble, the presence of an amino group in the compound is indicated because amines are organic bases that react with dilute acids to form ammonium salts that are usually water-soluble . However, this solubility test does not permit the distinction between weak and strong organic bases.

Solubility in Concentrated sulfuric acid

Many compounds that are too weakly basic or acidic to dissolve in dilute aqueous acid or base will dissolve in or react with concentrated H2SO4. Such solubility is often accompanied by the observation of a dark solution or the formation of a precipitate; any detectable reaction such as evolution of a gas or formation of precipitate is considered “solubility” in concentrated H2SO4. This behavior usually may be attributed to the presence of carbon- carbon pi bonds, oxygen, nitrogen, or sulfur in the unknown.

The solubility is normally due to reaction of one of these functional groups with the concentrated acid, which results in the formation of a salt that is soluble in the reagent. For example, an alkene adds the elements of sulfuric acid to form an alkyl hydrogen sulfate that is soluble in the acid, and an oxygen-containing compound becomes protonated in concentrated acid to form a soluble oxonium salt. Substances that exhibit this solubility behavior are termed “neutral” compounds.

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